May. 25, 2011

Animals frequently face the problems of feast and famine. Sometimes food is so abundant that an animal could not possibly eat more than a small fraction of what is available (the “feast”), and then there are extended periods when food is scarce (the “famine”). Many animals solve this problem by storing food for future use, a behavior called caching (pronounced “cashing”). Birds and squirrels are examples of animals that cache food. Sometimes there is a central cache where an animal stores all of its food in a single place. This is not a particularly good strategy, however, as competitors might find the location and steal the food reserves. Thus, most caching species have numerous caches, sometimes as many as a hundred, scattered throughout their home range. But, if the animal puts the collected food in many different places, how does the animal find its food again? The squirrel uses several different caching tactics. The following activity is adapted from Biology in a Box.

Grade Level: 3rdth – 6th grade
Subject Matter: Life Science

Activity Materials
Zipper bag containing squirrel caching game, as follows:
Round mat (about 12 inches in diameter) – you could also use placemat – this will serve as the “home range” of the squirrel
46 prominent colored chips (poker chips will work). On six of these chips, place a round sticker on one side - these will be the “acorn,” signifying the location of a food cache.
Four chips of a different color – to be used as sign posts in the home range (the mat)

Vocabulary
There are three basic ways in which animals relocate their multiple food caches to utilize stored food items during periods of food shortage, as follows:

1. Episodic Memory: In using this tactic, the animal remembers hiding each food item, just as you might remember a particular birthday party. When this tactic is used in finding multiple caches, it requires that the animal pay close attention to the path it takes to each cache location from where it has retrieved the food item.

2. Rule-based Search: In using this tactic, the individual follows certain rules in hiding its food. A rule-based search typically involves the use of “signposts,” such as the placement of food items only under rocks or only on the west side of trees. When performing a rule-based search, the animal only needs to remember the rule/signpost, but it may have to search multiple locations before finding its food. This is because there may be many more rocks, for instance, than those at which the individual cached food.

3. Re-foraging the Home Range: No memory is involved in this tactic. Instead, the animal searches the entire home range when looking for a meal during periods of famine. This is an energetically costly food caching strategy.

What to DoThis is a game for three or four (squirrels) students. One squirrel is the individual who will cache acorns around its home range. The cheating squirrels will have a chance to find the caches and steal the acorns before the owner gets a chance to retrieve them. If all of the acorns are not found, then the trees in the woodlot win because the squirrel has planted seeds that will germinate into tree seedlings the following spring.

1. While the game is being set up by the caching squirrel, the cheating squirrels need to engage in some other task while the caching squirrel is caching its food. The caching squirrel fills the matt with non-overlapping plastic chips of the dominant color without acorns underneath.

2. The caching squirrel then decides which caching tactic he or she will follow. Using the chosen caching tactic, the caching squirrel determines the cache locations for the six acorns, removes the non-acorn chips at the spots selected, and places the acorn chips there. Do the same for the four differently colored chips if you want to use them as sign-post chips (the rules-based cache technique).
Check to be sure that all chips are picture-side down.

3. Call the cheating squirrels back. Each gets a chance to turn over one chip. If that chip has an acorn under it, the individual takes the acorn for his or her stash and then can turn over another chip. This process continues until the 1st cheating squirrel does not find an acorn with a flip.

4. The second cheater gets a turn as above. The third cheater gets a turn as above. And so on.

5. The caching squirrel now returns to search for its caches, using the same protocol as described for cheaters.

6. The squirrel with the most acorn chips in his/her possession at the end of the game wins, though the trees win if more chips were not found than found.

7. Trade places so that the caching squirrel is a cheater and one of the cheaters is the cacher.

Topics for Science Class Discussion
Which squirrel (student) developed the most successful tactic? What was it?

Extended Activities and Links
Riechert SE, Leander RN, Lenhart SM. A role-playing exercise that demonstrates the process of evolution by natural selection: Caching squirrels in a world of pilferers. 2011. The American Biology Teacher 73:208-212

____________________________The National Institute for Mathematical and Biological Synthesis (NIMBioS) brings together researchers from around the world to collaborate across disciplinary boundaries to investigate solutions to basic and applied problems in the life sciences. NIMBioS is sponsored by the National Science Foundation, the U.S. Department of Homeland Security, and the U.S. Department of Agriculture with additional support from The University of Tennessee, Knoxville.